The long-term goal of this proposal is to understand the molecular mechanisms that regulate exocytosis in order to comprehend and develop therapies for devastating human diseases in which exocytosis is defective, such as diabetes and polycystic kidney disease. The overall objective of this work is to dissect the spatio-temporal regulation of exocytosis at a molecular level during development of epithelial cell polarity. This proposal focuses on three components of the trafficking machinery: the Exocyst (a putative vesicle tethering factor), Munc18c (a regulator of tethering/fusion machinery), and Syntaxin 4 (a component of the fusion machinery). Based upon results of preliminary studies, a working hypothesis has been developed for how polarized trafficking to the basal-lateral membrane is established. Cell-cell adhesion promotes assembly, movement and association of Exocyst with intercellular contact sites defined by E-cadherin-associated protein complexes;association of transport vesicles with these sites coincides with an interaction between an Exocyst subunit (Sec6) and Munc18c, and this in turn regulates the activity of Syntaxin 4, leading to membrane fusion. The specific aims of this study are to: 1) identify mechanisms that specify targeting patch assembly at sites of cell-cell adhesion;2) determine the functional significance of Sec6 binding to Munc18c in basal-lateral membrane trafficking;and 3) determine whether conformational changes in Sec6 regulate its association with Munc18c. The research plan to achieve these aims involves a combination of cell biology, biochemistry and molecular biology approaches. The significance of these studies is that they will define signaling pathways and specific interactions that couple an extracellular spatial cue (cell adhesion) to organization of components involved in membrane trafficking (vesicle docking/fusion machinery) leading to establishment of cell polarity. In the long term these studies will provide new insights into understanding the basis for abnormalities in membrane protein organizations characteristic of epithelial diseases. Also, because this proposal focuses on how information is transferred from vesicle to plasma membrane to initiate exocytosis, it has broad implications for many processes including antigen presentation, neurotransmission and hormone secretion.